Harassed and Threatened by a big corporate

HI: I signed a contract with a global office space provider in January 2014 but cancelled the contract on May 20th because it was not viable. Now after 2 months and a half they have come back to me with a invoice of an amount which is even more than the annual charges they were charging me before. they were charging me all this time and not informing me. when i went back to them they said its in the contract i can not cancel it. Their payment department has started harassing me with dire consequences etc. this while not even informing me that they were charging me all this while. What is the best course of action on this. Do I have legal rights in india as a customer and citizen to protect myself against such gross corporate malpractice. After receiving the threat letter I googled the companys name with customer complaint and to my surprise there are plenty. so this is their usual practice...which i find extremely shocking as their marketing and sales are excellent in selling the contract without ever informing the gullible customer of the fine lines. What is the best course of action on this. I can submit emails and files if required. Thanks!